1. Field of the Invention
The present invention relates to a technology for detecting toner concentration in initial developer contained in a developing device by a toner-concentration detecting unit and consuming toner in developer in the developing device based on a detected result, to decrease the toner concentration in the initial developer.
2. Description of the Related Art
Conventionally, there are known image forming apparatuses such as copiers, printers, and facsimiles that develop a latent image on a latent-image carrier using developer containing toner and magnetic carrier. In this type of image forming apparatuses, toner concentration in a developer is generally maintained to a certain level by supplying toner into a developing device based on a result of detecting a decrease in the toner concentration in the developer due to development by a toner-concentration detecting sensor.
As for the image forming apparatus that maintains the toner concentration to the certain level in the above manner, a device that calibrates a toner-concentration detecting sensor in the following manner is disclosed in, for example, Japanese Patent Application Laid-open No. 2007-225813.
More specifically, an initial developer with toner concentration controlled to a predetermined toner concentration is set in a developing device when it is shipped from a factory. When the image forming apparatus is delivered to a user and power is initially turned on, the toner concentration in the initial developer is detected by the toner-concentration detecting sensor. A control voltage to be input to the toner-concentration detecting sensor is controlled so that an output voltage from the toner-concentration detecting sensor at this time is set to a predetermined value. In this manner, the toner-concentration detecting sensor is calibrated so that a voltage of a predetermined value with respect to the predetermined toner concentration is output.
A sensor formed of a magnetic permeability sensor that detects a magnetic permeability is generally used as the toner-concentration detecting sensor. FIG. 11 is a graph representing an example of a relationship between an output voltage Vt of the toner-concentration detecting sensor formed of the magnetic permeability sensor and toner concentration in developer. As shown in FIG. 11, the output voltage Vt of the toner-concentration detecting sensor decreases with an increase in the toner concentration in the developer. This is because the density of magnetic carrier per unit volume decreases with an increase in the toner concentration in the developer and the magnetic permeability of the developer thereby decreases. Because the slope of the graph or the sensitivity of the toner-concentration detecting sensor varies depending on products, the sensitivity is measured before shipping and the measured sensitivity is stored in an integrated circuit (IC) chip incorporated in a recent toner-concentration detecting sensor.
A case of using an initial developer in which toner concentration is controlled to 7% by weight will be explained below. The measurement of the sensitivity of the toner-concentration detecting sensor is implemented by measuring output voltages Vt when magnetic samples are detected, the magnetic samples obtaining magnetic permeabilities the same as respective magnetic permeabilities of developers containing three types of toners with standard loose apparent density at the rate of 4%, 7%, and 10% respectively.
By calibrating the toner-concentration detecting sensor in the above manner, for example, a graph with the sensitivity as shown in FIG. 12 stored in the built-in IC chip in the toner-concentration detecting sensor can be used. An output voltage Vt corresponding to 7 wt % being the toner concentration in the initial developer is determined, and 2.7 volts is obtained. This value coincides with the value when it is calibrated.
In the developing device right after the toner-concentration detecting sensor is calibrated, the developer corresponds an initial developer which means no toner is consumed, however, the toner concentration in the initial developer is not always a value suitable for subsequent development. Because even if the toner concentration in the developer is the predetermined value, the environment causes the image density to be different. Specifically, even if the developer is stirred in the developing device, a frictional charge amount of the toner cannot be increased satisfactorily under high-temperature and high-humidity environment. The charge amount (Q/M) per unit weight of toner thereby decreases more than an ordinary value, and a toner adhesion amount per predetermined unit area of a latent image that has a predetermined voltage increases.
Consequently, development of a latent image using the developer without changing the toner concentration in the initial developer may cause a large amount of toner to adhere to the latent image and the image density to be therefore higher than a target density. Conversely, the toner is excessively frictionally charged with stirring of the developer under low-temperature low-humidity environment, and only a small amount of toner is thereby caused to adhere to the latent image, so that the image density may be lower than the target density.
There is known an image forming apparatus that controls toner concentration in the initial developer in the following manner after the toner-concentration detecting sensor is calibrated. Specifically, first, a preset reference toner patch image is formed, and a toner adhesion amount (image density) per unit area of the reference toner patch image is detected by a reflective photosensor or the like. If the result of detection is lower than a target value (low image density), then toner is supplied into a developing device, and then a reference toner patch image is again formed. Thereafter, toner supply, formation of a reference toner patch image, and detection of a toner adhesion amount are repeated until the toner adhesion amount of the reference toner patch image reaches the target value.
On the other hand, if the toner adhesion amount of the reference toner patch image is higher than the target value (high image density), a solid toner image for forcible consumption of toner is formed and the toner in the initial developer is forcibly consumed, and then a reference toner patch image is formed again. Thereafter, formation of a solid toner image, formation of a reference toner patch image, and detection of a toner adhesion amount are repeated until the toner adhesion amount of the reference toner patch image reaches the target value.
By controlling the toner concentration in the initial developer in the above manner, development can be preformed using the developer with the toner concentration matching the environment, from a first sheet of initial prints.
The control of the toner concentration in the developer is generally performed in such a range that an output voltage Vt from the toner-concentration detecting sensor after being calibrated is not increased higher than a predetermined upper limit nor decreased lower than a predetermined lower limit. This is based on the reason as explained below. If the toner concentration is increased too high, a phenomenon called “background stains” is caused by the fact that toner is made to adhere to a background portion of a latent-image carrier. If the toner concentration is decreased too low, a phenomenon called “carrier adhesion” is caused by the fact that magnetic carrier of the developer in the developing device is made to adhere to the latent-image carrier. To prevent the background stains and the carrier adhesion, the output voltage Vt is maintained in the range from the lower limit to the upper limit. When the toner concentration in the initial developer is controlled in the above manner, the toner concentration is also controlled by maintaining the output voltage Vt in the range from the lower limit to the upper limit.
Referring to FIG. 12, “7-β” represents the lower limit of the toner concentration in the developer, and if the toner concentration decreases more than this, the carrier adhesion may be caused. A voltage corresponding to the lower limit of the toner concentration is an upper limit Vt1 of the output voltage Vt. Further, “7+α” represents the upper limit of the toner concentration in the initial developer, and if the toner concentration increases more than this, the background stains may be caused. A voltage corresponding to the upper limit of the toner concentration is a lower limit Vt2 of the output voltage Vt. The upper limit Vt1 and the lower limit Vt2 are stored in the built-in IC chip in the toner-concentration detecting sensor, together with the graph of FIG. 12 representing the sensitivity of the toner-concentration detecting sensor. By using the upper limit Vt1 and the lower limit Vt2 for controlling the toner concentration, it is possible to prevent the background stains and the carrier adhesion.
However, it is found that even if the upper limit Vt1 is used, the carrier adhesion may be caused upon initial printing due to variation in a loose apparent density of toner contained in the initial developer, and that the carrier adhesion may damage the latent-image carrier such as a photosensitive element.
The reason is explained below.
Generally, even if toner is manufactured in the same method and under the same environment, a loose apparent density (volume per unit mass) of the toner varies with each product. Meanwhile, the toner-concentration detecting sensor formed of the magnetic permeability sensor does not detect the toner concentration itself in the developer but detects carrier density in the developer (bulk density of developer). Even if the toner concentration in the developer (a weight ratio of toner to magnetic carrier) is constant, the carrier density in the developer may become different if the loose apparent density of toner is different. Therefore, even if the developer with the same toner concentration is used, an output value from the toner-concentration detecting sensor becomes different if the loose apparent density of toner is different.
The slope of the graph for sensitivity shown in FIG. 12 represents a case where toner with a standard loose apparent density is used, and thus the loose apparent density of toner actually contained in the initial developer is not always a standard value. If the loose apparent density of toner in the initial developer is higher than the standard value, or if the volume of toner per unit mass is comparatively small, a more gentle slope indicated by the solid line as shown in FIG. 13 is formed in the graph of the output voltage Vt of the toner-concentration detecting sensor that uses the initial developer as an object to be detected. When the loose apparent density of the toner is high, a change in the volume of the toner in the developer becomes comparatively small caused by a change in toner concentration, and a rate of change of the magnetic permeability with respect to the change in the toner concentration becomes thereby comparatively small.
In this case, the upper limit Vt1 of the output voltage Vt stored in the toner-concentration detecting sensor is “7-β-γ” lower than “7-β” which is the lower limit of the toner concentration in the graph indicated by the solid line of FIG. 13. Specifically, despite setting the output voltage Vt to be lower than the upper limit Vt1, the toner concentration becomes lower than the lower limit. This causes the carrier adhesion.
When a two-component developer is used to repeatedly develop an electrostatic latent image, toner in the developer is consumed and the toner concentration thereby fluctuates. Therefore, to obtain stable images upon printing, it is necessary to supply toner as required and minimize the fluctuation. Generally, when a toner supply amount is controlled, an image forming apparatus such as a copier includes a permeability detecting sensor, a flowability detecting sensor, an image-density detecting sensor, a bulk-density detecting sensor, and a like. A recent mainstream of these sensors is to use the image-density detecting sensor or a combination of the image-density detecting sensor and the magnetic permeability sensor (a type of bulk-density sensor). The control of the toner concentration using the image-density detecting sensor is performed by a system of controlling a toner supply amount by developing a fixed image pattern on the image carrier and detecting an image density from the reflected light. The control of the toner concentration using the combination of the image-density detecting sensor and the magnetic permeability sensor is performed by a system of controlling a toner supply amount by changing a target value of the magnetic permeability sensor based on the density of an image pattern.
An invention disclosed in Japanese Patent Application Laid-open No. 2005-346102 has less fluctuation in the bulk density of developer and can thereby stably control the toner concentration even when the developer is used under high stress, and, therefore, has solved such a defect that the toner concentration is unstably controlled caused by fluctuation in the bulk density of the developer when the developer is used under high stress.
However, there are some problems as follows in controlling an output of the initial developer of the magnetic permeability sensor.
An output of the magnetic permeability sensor for the initial developer is generally controlled based on reference toner concentration. Therefore, in a case of other toner concentrations, an output is calculated from a relationship between an output of the magnetic permeability sensor based on the reference toner concentration and toner concentration. However, the output of the magnetic permeability sensor has such a property that the output decreases as carrier being a magnetic body is far from the magnetic permeability sensor or as carrier particles are sparsely distributed. Consequently, if the carrier particles separate from the upper portion of the toner-concentration detecting sensor due to the decrease in the bulk density of the developer or are sparsely distributed, this results in an erroneous detection that the output decreases and the toner concentration increases, although the toner concentration is not changed. Conversely, if the carrier particles become dense in the upper portion of the toner-concentration detecting sensor because of an increase in the bulk density of the developer, this results in an erroneous detection that the output increases and the toner concentration decreases, although the toner concentration is not changed.
This is because the bulk density of carrier in the developer is different depending on variation due to its manufacture and the bulk density in the developer varies to make different the state of the carrier particles in the upper portion of the toner-concentration detecting sensor. To minimize the variation, the output of the toner-concentration detecting sensor is controlled and corrected using the initial developer based on the same toner concentration. As a factor that the erroneous detection occurs with the magnetic permeability sensor, a relationship between the output of the magnetic permeability sensor and the toner concentration is the same when the reference toner concentration is used, but the relationship between the two is different when the toner concentration changes.
Because the toner concentration changes due to the environment and the use pattern, appropriate toner concentration needs to be provided for the developing device. This is because toner scattering or background stains may occur if the toner concentration becomes too high while an image is formed abnormally due to insufficient supply of the developer if the toner concentration becomes too low. The upper limit and the lower limit of the toner concentration are controlled by an output value of the toner-concentration detecting sensor not to cause these defects.
However, the variation cannot be suppressed satisfactorily only by controlling the output of the toner-concentration detecting sensor when the initial developer is used because the upper and lower limits of the toner concentration to be controlled are different depending on the bulk density of the carrier in the initial developer. Because of this, if the upper limit of the toner concentration becomes high, toner scattering or background stains may occur, or the image density may be insufficient if the upper limit thereof becomes low.
If the lower limit of the toner concentration becomes high, the toner concentration cannot be decreased, so that the image density may be too high. If the lower limit of the toner concentration becomes low, the bulk of the developer becomes too low, so that the developer is unstably sucked up to a developing element to cause an image to be abnormal.
Besides, the invention disclosed in Japanese Patent Application Laid-open No. 2005-346102 does not mention about defects that the upper and lower limits of the toner concentration are unstably controlled caused by fluctuation in the bulk density of the developer due to a difference in bulk density of carriers in initial developers.